Electrostatic spraying apparatus, especially for use in painting workpieces, generally comprise a duct for conveying gas-borne powder and means for electrostatically charging the powder, whereby it adheres to the workpiece. Typically a corona discharge needle electrode is used to charge the powder.
Corona charging of paint powder is not without its shortcomings although it is a preferred method of spray coating for a majority of powder coaters. Problems associated with Faraday Cages, back ionisation orange peel (pitting) and overcharging have been well documented but advantages of consistency, good charge transfer and fast powder deposition largely outweigh the above-mentioned shortcomings.
The effects of such shortcomings may be reduced by spray nozzle design or by addition of earth robbing electrodes behind the spray nozzle, as well as by careful setting of the spray apparatus position and operating parameters.
Significant improvements can also be achieved by controlling the discharge current instead of the discharge voltage which has proved its worth over 25 years but surprisingly this method is still the exception rather than the rule. With this system the maximum discharge current (.mu.A) is limited to a value determined by the operator and the discharge voltage (kV) is allowed to float. The result is that as the spray apparatus approaches the workpiece and the set discharge current is reached and stabilises, so the output voltage automatically reduces to relatively low levels, thus limiting the charge when the apparatus is close to the workpiece. This is of great benefit in maintaining charge consistency, penetrating Faraday Cages and reducing back ionisation and orange peel appearance.
With conventional voltage control, where the discharge voltage is set by the operator, the current rises exponentially as the spray apparatus approaches the product which has the opposite effect to that of using current control and can also cause severe "over charging" of the powder and surrounding air and can give rise to potentially dangerous sparks, although safety cut out circuits are usually employed.
Despite the many advantages of current controlled corona charging, it is still not perfect because the output voltage is allowed to float, when the spray apparatus is pulled away from the workpiece the current can fall below its "constant" or controlled setting and the voltage can rise to the maximum available from the generator, usually 80+kV. In some cases this may be too high and create strong field lines, with the resulting Faraday Cage difficulties. As the gun approaches the product, the air space between the gun and the product obviously reduces and will therefore accept a progressively smaller charge from the corona needle in terms of free ions. It is the free ions which charge the air molecules and then transfer the charge to the powder. The closer the spray apparatus is to the workpiece so the greater the proportion of free ions which are attracted to the surface of the workpiece as opposed to dissipating in the surrounding air, or being neutralised by metal walls of the spray booth. This high charge on the surface of the powder is the main cause of the orange peel effect.
An aim of a first aspect of the present invention is to provide an improved method of controlling operation of powder spray coating apparatus whereby the above-mentioned shortcomings of prior art methods may be overcome or at least reduced in effect.
In conventional electrostatic spraying apparatus, the flow of powder from a container to the means for electrostatically charging the powder is controlled by compressed air. As the spray apparatus approaches a workpiece, the surface area which is sprayed reduces and hence the volume of powder paint required also reduces. In practice, as the spray apparatus approaches the workpiece, an operator will manually reduce the volume of powder flowing to the electrostatic charging means by, for example, reducing the pressure of the compressed air. However, since the volume of powder sprayed is controlled manually, this can lead to irreproducible coating results.
An aim of a second aspect of the present invention is to provide an electrostatic spraying apparatus which overcomes the afore-mentioned problem.